Hi there @Cyrinus, it depends on the pet. Click on one of your placed pets to go to their hoard page. From there, click the button that says "View Options" (it'll be above "Move Pixpet"). Now on the pet's options page, underneath the image of the pet, you should see the Evolution Options. Here you can see what grade is required for their evobar, as well as what level the pet needs to be for evolution. If you don't see the evolution options, you may have clicked a pet that doesn't evolve.

Let me know if anything I said was hard to understand or if you have further questions ^^